Repointing looks simple from the pavement. Rake out the old joints, push in new mortar, brush it off and the wall looks tidier. The trouble starts when the new mortar is harder, denser or less breathable than the brickwork it has been put into.
We see this a lot on London houses, especially Victorian and Edwardian terraces with soft yellow stock bricks. A strong cement mix may look neat for the first winter, then the brick faces start flaking, damp patches appear indoors, or cracks open around window heads and parapets. At that point, the repointing has stopped being cosmetic and has become a building defect.
If you are asking what happens if you repoint with the wrong mortar, the short answer is that the wall can no longer dry and move as it should. The long answer is worth understanding before anyone cuts into your brickwork.
Why the Wrong Mortar Matters on Older London Brickwork
Mortar is not just glue between bricks. On older solid-wall properties, it acts as the softer, breathable part of the wall. Rainwater gets into the outer face during bad weather, then the wall dries out again through the joints. The mortar is meant to take wear before the brick does.
Many Kensington and Chelsea properties were built with lime-based mortar, not modern hard cement. Lime mortar is softer and more vapour-permeable, which suits old London stock brick, shallow foundations and solid external walls. If a dense cement mortar is packed into those joints, the moisture often has nowhere sensible to go.
The usual result is spalling, where the outer face of the brick pops or flakes away. We also see trapped damp, salt staining, hairline cracking and patches where the joint has separated from the brick. Around chimney stacks, parapets and garden walls, the damage can move quickly because those areas are hit by rain from several directions.
Modern mortar is not always wrong. Newer cavity-wall brickwork may use a cement-based mortar specified to BS EN 998-2, and that can be perfectly suitable. The mistake is assuming one strong mix fits every wall. It does not.
Inspect the Brickwork Before Any Raking Out Starts
Start by looking at the age, brick type and condition of the wall. Soft yellow stock bricks, red rubbers around windows, handmade bricks and uneven old joints all point towards a need for a compatible lime or lime-rich mortar. Very hard blue engineering bricks or modern facing bricks may need a different approach.
Check for spalled brick faces, hollow-sounding patches, stepped cracks, damp internal plaster and white salts on the brickwork. These signs tell us the wall may already be holding moisture or under stress. If the wall is bulging, the chimney stack is leaning, or brickwork is loose above a public footpath, do not treat it as a DIY pointing job. Get a professional inspection first.
On many London homes, there is more than one mortar on the same elevation because patch repairs have been done over decades. We often find original lime mortar low down, hard grey cement around a boiler flue, and a sandy modern patch by a window repair. Each area needs judging properly, not just skimmed over to make it all look uniform.
Identify the Existing Mortar and Choose a Compatible Mix
The right mortar depends on the brick, exposure and existing building fabric. For older London stock brickwork, a lime mortar is often the better choice because it remains softer than the brick and lets the wall breathe. The exact mix can vary, but common choices include lime putty mortars for sheltered historic brickwork or NHL lime mortars where more weather resistance is needed.
Colour matters, but performance matters more. We match sand colour, grain size and joint finish because a badly matched repair stands out, but we do not use a harder mix just because it sets quickly. A bright grey cement joint on warm yellow brick is usually a warning sign as well as an eyesore.
If the building is listed or in a conservation area, which is common around Kensington, Chelsea, Holland Park and parts of Notting Hill, changes to external brickwork may need extra care and sometimes consent. Even where formal consent is not needed, poor mortar choice can reduce the value and character of the building. For a small area, a mortar analysis or sample panel may cost roughly £80-£200, but it can save thousands in avoidable brick repairs.
Rake Out the Failed Mortar Without Damaging the Brick Arrises
Old mortar should be removed to a sensible depth, often around 15-25mm, or deeper where the joint has failed. The aim is to create enough depth for the new mortar to key properly. Smearing a thin layer over old hard cement is not repointing, it is a temporary cover-up.
Raking out must be controlled. On soft brickwork, aggressive angle grinder work can scar the brick arrises, widen the joints and leave the wall looking butchered. Hand tools, plugging chisels and careful dust control are often safer on period brickwork, especially around window arches and decorative details.
Where hard cement has already been used, removal can be slow. It may pull brick faces away if rushed, so we usually test a small area first. On occupied homes and commercial premises, we also plan dust sheeting, access and waste removal properly because mortar dust travels into sash boxes, vents and neighbouring areas very easily.
Dampen the Joints and Pack the New Mortar Firmly
Dry brick will suck moisture out of fresh mortar too quickly. That gives you weak, crumbly joints and poor curing. Before repointing, the joints should be cleaned of dust and lightly dampened so the mortar cures steadily rather than drying on contact.
The mortar should be pressed firmly into the joint in layers where needed, not just wiped across the face. Deep open joints, chimney brickwork and exposed parapets often need more careful packing because voids behind the face allow rainwater to sit in the wall.
We also avoid doing lime pointing in frost, heavy rain or very hot direct sun. Lime needs protection while it cures. In London, spring and early autumn are often better for larger repointing work than the middle of a wet January or a hot July week on a south-facing wall.
Finish the Joint to Shed Water Properly
The joint profile affects how the wall handles rain. A flush, slightly recessed or weather-struck finish may be suitable depending on the original work and exposure. Deep ribbon pointing on soft brick can trap water on ledges and look completely wrong on a period elevation.
Good pointing is compact, clean and sympathetic to the brick. It should not smear mortar over the face, block air bricks, bridge damp proof courses or cover weep paths. Around basement lightwells, garden walls and lower front elevations, those small details make a noticeable difference to damp problems.
For conservation work, matching the original joint style matters. A neat modern joint on one part of a Victorian frontage can stand out more than the old weathering ever did. We usually create a small sample area first so the customer can see the colour and finish once it starts drying.
Let the Mortar Cure and Check the Wall After Weathering
Mortar changes colour as it cures, especially lime mortar. It may look darker when first applied, then lighten over days and weeks. Judging the colour on the first afternoon is a common mistake.
Fresh lime work should be protected from rapid drying, driving rain and frost. That can mean hessian, sheeting or staged work depending on the elevation. If scaffold is up, protection is easier to manage, but it still needs someone checking it rather than leaving the wall exposed all weekend.
After the first spell of wet weather, look for joints that have shrunk back, cracked or washed out. A properly specified and applied mortar should settle in without pulling away from the brick. If problems show quickly, it is better to correct a small section than wait until a whole elevation has failed.
Costs, Timing and When to Call a Professional
Repointing costs vary because access, mortar type and brick condition make a big difference. As a guide in London, straightforward repointing on accessible brickwork might sit around £60-£110 per square metre. Lime repointing on older or more delicate brickwork is often around £90-£160 per square metre because it takes longer to rake out, apply and cure properly.
Chimney stack repointing commonly costs around £450-£1,200 depending on height, condition and access. Scaffold can be a large part of the bill. A front elevation scaffold on a London terrace can easily add £800-£2,500, more if pavement licences, awkward access or multiple lifts are needed. Brick replacement for spalled units may add roughly £8-£20 per brick on small repairs, with specialist matching bricks costing more.
DIY may be reasonable for a small, low garden wall if you understand the mortar and can work safely. It is not sensible for chimney stacks, parapets, upper floors, bulging walls, listed buildings, or any area over a pavement or neighbour’s property. Working at height must be planned properly under the Work at Height Regulations 2005, and loose masonry can injure people below. Safety-related work should always be professionally inspected.
There are also knock-on trades to think about. Failed pointing can let water into roof abutments, loft spaces, internal plaster and timber. If we are investigating damp after repointing, our roofers may check flashings and parapets, and our decorators may need to make good internal finishes only after the wall has properly dried. Repainting damp plaster too early just seals the problem in.
Timing matters. Exterior lime work is best planned when temperatures are mild and frost is unlikely. Many landlords book repointing between tenancies to avoid scaffold blocking windows while tenants are in occupation. For shops and offices, we normally plan noisy raking-out work around opening hours where possible, especially on busy streets in Kensington and Chelsea.
Frequently Asked Questions
Q1: Can wrong mortar really damage bricks?
Yes. If the mortar is harder or less breathable than the brick, moisture and salts can be forced through the brick face instead of the joint. That often leads to spalling, flaking and early brick failure.
Q2: How can I tell if my house has been repointed with cement?
Hard cement pointing is often grey, very dense and difficult to scratch with a key or nail. You may also see cracks along the edge of the joint where it has pulled away from softer brick.
Q3: Does all old mortar need to be replaced with lime?
Not always, but many older London solid-wall properties perform better with lime or lime-rich mortar. The brick type, exposure and existing mortar should be checked before deciding.
Q4: How long should proper repointing last?
Good repointing can last decades if the right mortar is used and the wall is not suffering from another defect such as a leaking gutter or failed flashing. Poor cement pointing on soft brick can cause visible damage within a few winters.
Q5: Should I remove bad pointing myself?
Only on small, low-risk areas where the brickwork is sound and access is safe. For upper walls, chimneys, listed buildings, cracked brickwork or damp problems, call a professional before cutting anything out.
The main lesson is simple: mortar should suit the brickwork, not overpower it. If the wrong mortar has already been used, catching it early can prevent spalled bricks, damp rooms and expensive elevation repairs later on. Our team at Kensington Maintenance Services Ltd regularly deals with repointing, brick repairs and related exterior maintenance on London properties, so if you want the wall checked properly or need a practical price for the work, you can get a quote or contact our specialists and we will talk you through the sensible next step.